1. Field of the Invention
This invention relates to improvements in a metallic catalyst carrier of a catalytic converter or the like which is disposed in an exhaust passage of an internal combustion engine in order to purify exhaust gas discharged from the engine, and more particularly to the improvements for achieving early activation of the catalyst carried on the catalyst carrier and raising a conversion efficiency of the catalyst.
2. Description of the Prior Art
Hitherto a catalytic converter including a metallic catalyst carrier encased in a shell has been used upon being disposed in an exhaust gas passage of an internal combustion engine in order to purify exhaust gas discharged from the engine, i.e., to convert noxious exhaust gas components such as CO, HC, NOx and the like into harmless gas. The metallic catalyst carrier carries thereon a catalyst meterial to form a catalytic element. An example of such a metallic catalyst carrier is constituted of a plurality of corrugated metal sheets and a plurality of flat metal sheets and produced by alternately putting each corrugated metal sheet and each plate metal sheet one upon another, as disclosed in an International Publication No. WO 97/45200.
In such a metallic catalyst carrier, each corrugated metal sheet has a section which is located between gas inlet and outlet sections and has corrugations smaller than normal corrugations thereby reducing contact portions between the corrugated and flat metal sheets. This is intended to enlarge the surface area of the catalyst carrier and therefore increases the surface area of the catalyst material to be exposed to exhaust gas, thus improving a conversion efficiency for the noxious gas after warning-up of the engine.
Now, in order to effectively convert the noxious exhaust gas components such as CO, HC, NOx and the like into harmless gas, it is essential to promptly activate the catalyst material during engine warming-up in addition to raise the conversion efficiency after engine warming-up. For the purpose of prompt or early activation of the catalyst, it is effective to reduce the heat mass or capacity of the gas inlet section of the catalytic element. This is because early activation of the gas inlet section of the catalytic element promotes activation of the gas outlet section of the catalytic element under the action of reaction heat generated at the gas inlet section. However, reducing the heat capacity of the gas inlet section results in reducing the heat capacity of the whole catalytic element, and therefore there arises such a problem that the conversion efficiency is unavoidably lowered after engine warming-up. In other words, with the above-discussed metallic catalyst carrier in which the surface area of the catalyst carrier is increased by reducing the contact portions between the corrugated and flat metal sheets, there exists a certain limit for raising the conversion efficiency after engine warming-up, and therefore a further improvement in conversion efficiency is required. Additionally, it is difficult to reduce the heat capacity of the metallic catalyst carrier for the purpose of intending early activation of the catalyst material.